Annual immunization against influenza infection is one of the largest coordinated international public health efforts. Current flu vaccination strategies primarily elicit protection by the generating long lasting type-specific neutralizing IgG anti-hemagglutinin (HA) antibodies that bind to molecularly similar influenza subtypes. This phenomenon is termed antibody mediated heterosubtypic immunity (amHSI), and a major reason for the success of seasonal influenza vaccination. MF59 is a squaline-oil adjuvant recently approved for seasonal influenza vaccination in indivduals ?65 years of age. Our preliminary data suggests that MF59 increases amHSI in mice, ferrets and human subjects. Thus the primary goal of this Project is to elucidate and model the mechanisms of by which MF59 adjuvanted seasonal influenza vaccine increase B cell mediated amHSI at the immunoglobulin heavy chain DNA and protein repertoire level. Aim 1: To test the hypothesis that MF59 adjuvant increases the breadth, depth and molecular sequence diversity in the IgG repertoire after influenza vaccination. Aim 2: To build and validate an age-dependent branching process model of heterosubtypic immunity coverage induced by adjuvanted influenza vaccine. Aim 3: To model and identify the mechanisms responsible for MF59 adjuvanted influenza vaccine induced anti-HA IgG repertoire evolution and amHSI generation in human vaccine recipients. This proposal addresses a highly significant issue in public health, how to optimize the protection of the influenza vaccine using vaccine adjuvants to increase the cross-strain reactivity of the resulting mixture of IgG anti-HA antibodies. It also addresses a significant gap in scientific methods for reconstructing Ig sequence lineages resulting from hyperaccellerated somatic mutation within germinal center reactions. We will create age dependent branching process models that will provide mechanistic insight into how the adjuvant and intradermal vaccination alter the molecular diversity of antibody-mediated HSI. These models will be first developed using mice vaccinated with MF59 adjuvanted influenza vaccine in Aim 1, and then extended to human subjects in Aim 2. If successful, this work will provide a general framework for modeling the molecular processes involved in the generation of amHSI.